One of the problems associated with Joule-Thomson coolers is their inability to achieve temperature stabilities of less than 1.degree. K. per minute in the presence of a varying temperature, environmental pressure, and heat loads.
Prior art Joule-Thomson (J-T) coolers, particularly of the high efficiency miniature "demand-flow" type, have been used for more than 20 years to cool infrared (IR) detectors and other temperature-sensitive instruments in a multiplicity of military, commercial, and scientific applications. These J-T coolers are ideal for a relatively short duration, e.g., less than 10 hours, in applications such as missile, infrared guidance sensors, and instruments on scientific balloon flights. The temperature achieved by these coolers is directly related to the pressure at which the gas is exhausted from the device. If the exhaust ambient pressure varies, for example, due to atmospheric pressure variations which occur due to altitude changes in a missile or balloon flight, then the cooling temperature will also vary accordingly. In addition, while demand-flow J-T valves are able to accommodate variations in heat load, for example, due to varying the power dissipation of the device being cooled, or a changing parasitic heat leak due to a varying environmental temperature, temperature fluctuations of about 1.degree. to 5.degree. K. are still common.
Ultra-high temperature stabilities are required for many applications. Some of these applications include solid state tunable diode lasers (TDLs), whose output frequencies are extremely sensitive to temperature. TDLs require temperature stabilities on the order of 0.1 mK per minute or better.
TDLs are often flown on earth balloon flights as integral parts of infrared spectrometers which monitor constituents in the atmosphere. In typical laboratory and earth balloon flight applications, TDLs must be cooled to between 80.degree. K. and 90.degree. K. to operate effectively and are cooled by immersion of a cold finger into a large liquid nitrogen dewar. These systems are large and heavy, and the cost of refilling the liquid nitrogen is considerable.
The cooler size and mass can be reduced considerably, and the liquid nitrogen cost eliminated completely, by using a miniature J-T blow-down system which requires only a relatively small tank of room temperature gaseous nitrogen, instead of a dewar of liquid nitrogen. However, the problem with conventional J-T systems is their inability to achieve the required temperature stability.
An example of a cryogenic refrigeration system is disclosed in U.S. Pat. No. 3,728,868 by Longsworth. Longsworth discloses a cryogenic refrigeration system having a low thermal mass cryostat coupled to a sensing element which controls a valve that, in turn, regulates fluid flow through a J-T orifice. The sensing element is interposed between a warm end of the cryostat and the J-T orifice. The level of liquid in the system rises to a location near the cold extremity of the sensing element, and this level is the operative control condition. Variations in fluid level about this point adjust for changes in gas pressure, ambient temperature, heat load, and working fluid.
The Hingst U.S. Pat. No. 4,819,451, discloses a countercurrent heat exchanger located in a forward flow conduit in a dewar vessel located in a cryostatic device, used for cooling an infrared detector, based on the J-T effect. To reduce the heat load of an infrared detector, an insulating layer is arranged between the dewar vessel and a base. The cooling power of the J-T process is improved upon by having an inlet of the forward flow conduit cooled by Peltier elements. Other pertinent U.S. Pats. are U.S. Pat. No. 4,570,457, by Campbell; U.S. Pat. No. 4,606,201, by Longsworth; U.S. Pat. No. 4,569,210, by Albangnac; and U.S. Pat. No. 4,468,935, by Albangnac.
As can be appreciated, there exists a need for an improved J-T cooler that is able to operate at stable temperatures in the presence of varying temperature, atmospheric pressure, and heat loads.